(1) Field of the Invention
The present invention relates to a multi-wavelength light source for generating a plurality of lights having different wavelengths, and in particular, to a multi-wavelength light source to stably generate lights having arbitrary wavelengths by utilizing an acousto-optic tunable filter (AOTF).
(2) Related Art
With the explosive increase of communication capacities due to the wide-spread of Internet and the like, the introduction of a wavelength division multiplexed (WDM) optical communication system has been proceeded, and the transmission capacities of WDM optical communication systems have now reached a level of 1 tera-bit/second by the densification of communication wavelengths and the broadened bands and the like. In order to construct such a high-density WDM optical communication system, it is required to prepare a number of light sources corresponding to the number of wavelengths of WDM signal lights, thereby resulting in a considerable problem of cost. Thus, there have been eagerly desired inexpensive multi-wavelength light sources. Among these light sources, a multi-wavelength oscillation laser adopting rare earth element-doped fibers has been studied from an early stage, because of the simplified constitution thereof.
As the conventional multi-wavelength oscillation laser, various types of multi-wavelength light sources have been tried by combining optical fiber amplifiers utilizing erbium-doped fibers (EDF""s) with periodic filters such as etalons, as shown in FIG. 16. However, in such a multi-wavelength oscillation laser, there is caused inter-mode competition due to the wide homogeneous broadening widths possessed by EDF""s as shown in FIG. 17, resulted in that stable multi-wavelength oscillation cannot be obtained. Conventionally, in order to obtain stable multi-wavelength oscillation by means of erbium-doped fibers, there has been adopted a method to narrow the homogeneous broadening widths by cooling the erbium-doped fibers.
Further, there have been proposed those techniques for utilizing a frequency shifter so as to realize the multi-wavelength oscillation at the room temperature by utilizing erbium-doped fibers, such as in the articles: H. Sasamori et al., xe2x80x9cMulti-wavelength Erbium-doped Fiber Ring Light Source with Fiber Gratingxe2x80x9d, Technical Digest of Optical Amplifiers and Their Applications (OAA""97), paper WC3, pp. 235-238, 1997; A. Bellemare et al., xe2x80x9cMultifrequency erbium-doped fiber ring lasers anchored on the ITU frequency gridxe2x80x9d, Optical Fiber Conference (OFC""99), paper TuB5, pp. 16-18, 1999; and Seung-Kwan Kim et al., xe2x80x9cWideband multiwavelength erbium-doped fiber lasersxe2x80x9d, Optical Fiber Conference (OFC2000), paper ThA3, 2000. Specifically, for example, as shown in FIG. 18, an acousto-optic frequency shifter (AOFS) utilizing acousto-optic effect is inserted into a resonator, such that the frequency of light circulating through the resonator is shifted every circulation by a slight amount so as to prevent the stable oscillation at a single wavelength. As a result, simultaneous oscillations at multi-wavelengths can be obtained without inter-mode competition. Note, the acousto-optic frequency shifter utilizing acousto-optic effect is to collectively convert all of the wavelengths by utilizing Bragg reflection or the like.
In a case of multi-wavelength light source utilizing such a frequency shifter, there are caused variations among peak powers of respective wavelengths upon multi-wavelength oscillation, due to gain differences of the erbium-doped fiber, differences of resonator losses or the like, with respect to the respective wavelengths. This requires a device for correcting such variations. As such, in the conventional multi-wavelength light source, for example, as shown in FIG. 19, it has been attempted to flatten the optical intensity distribution, by introducing an equalizer utilizing such as variable attenuators corresponding to the respective wavelengths, in addition to the erbium-doped fiber and the periodic filter such as an etalon.
However, the aforementioned conventional multi-wavelength light source has such defects that the constitution thereof is complicated and that the adjustment of optical powers at respective wavelengths is difficult. Further, the characteristic of the periodic filter is fixed, thereby causing a problem in that it is impossible to obtain oscillation at arbitrary wavelengths.
The present invention has been achieved in view of the those points as described above, and it is therefore an object of the present invention to provide a multi-wavelength light source capable of stably generating a plurality of oscillated lights of arbitrary wavelengths by a simplified constitution. It is a further object of the present invention to provide a multi-wavelength light source capable of performing such as the adjustment of optical powers at respective wavelengths and the switching of wavelengths at high-speed and at ease.
To achieve the above objects, a multi-wavelength light source according to the present invention provided with an optical amplifier on an optical path of a resonator, for generating lights of plural wavelengths that have been propagated through the resonator and have oscillated, comprises an acousto-optic tunable filter on the optical path of the resonator, wherein the acousto-optic tunable filter selects lights of plural wavelengths from lights input thereto corresponding to frequencies of plural surface acoustic waves at different frequencies, respectively, and shifts the frequencies of the selected lights to output the frequency shifted lights to the optical path.
Conventionally, as described above the acousto-optic frequency shifter (AOFS) that has been used to realize a multi-wavelength light source is to utilize the Bragg reflection or the like, and has a function to collectively shift frequencies of lights in all of the bands. Contrary, the acousto-optic tunable filter to be used in the multi-wavelength light source of the present invention is to utilize the TE/TM mode conversion, and has a strong wavelength-selecting characteristic. Further, the frequency shift in the acousto-optic tunable filter has a characteristic to independently act on the respective selected lights so as to shift the frequencies of the selected lights by amounts of the frequencies of the corresponding surface acoustic waves, respectively.
According to the multi-wavelength light source using such an acousto-optic tunable filter, the lights of plural wavelengths selected at the acousto-optic tunable filter are propagated through the resonator, and the oscillation in a single mode within the resonator is suppressed by the frequency shift caused in each of the selected lights. As a result, it becomes possible to realize the oscillation of lights at multi-wavelengths without inter-mode competition.
Further, although the light wavelengths obtained in the conventional multi-wavelength light source are fixed, the multi-wavelength light source of the present invention is capable of operating at arbitrary multi-wavelengths corresponding to such as frequencies of RF signals to be applied to the acousto-optic tunable filter, and also capable of switching the required number of wavelengths and/or the wavelengths at higher-speed.
Moreover, as a specific constitution of the multi-wavelength light source of the present invention, the resonator may be a ring resonator or a Fabry-Perot resonator. In a case of the ring resonator, it becomes possible to realize the more stable multi-wavelength oscillation by giving the ring resonator a unidirectional constitution having an optical isolator on the optical path. Further, the multi-wavelength light source may be provided with an optical filter having a periodic transmission wavelength characteristic, on the optical path of the resonator. By providing the optical filter, it becomes possible to obtain oscillated lights of multi-wavelengths of narrower bands, and to realize a light source capable of arbitrary multi-wavelength oscillation on a discrete wavelength grid.
The multi-wavelength light source of the present invention preferably comprises an optical power control section for controlling an optical power of each of the lights of plural wavelengths that have been propagated through the resonator and have oscillated. Specifically, the optical power control section may control the optical powers at respective wavelengths by varying the intensities of the plural surface acoustic waves at different frequencies generated at the acousto-optic tunable filter, or by varying losses corresponding to oscillation wavelengths of an equalizer inserted into the optical path of the resonator, respectively. Moreover, the optical power control section may control the optical powers at respective wavelengths in accordance with a detection result by a monitoring section for detecting powers of the lights of plural wavelengths or correction values previously obtained corresponding to combinations of oscillation wavelengths.
By providing such an optical power control section, it becomes possible to control the optical powers at the respective oscillation wavelengths. Particularly, if the intensities of the surface acoustic waves in the acousto-optic tunable filter are controlled, it becomes possible to perform high-speed tuning of optical powers at respective wavelengths.
In the aforementioned multi-wavelength light source, the acousto-optic tunable filter may be constituted such that a plurality of acousto-optic tunable filter elements are connected to each other in multi-stages, so that the sum of frequency shifts of selected lights in the respective acousto-optic tunable filter elements becomes a value other than zero. By using such an acousto-optic tunable filter of multi-staged constitution, it becomes possible to narrow bandwidths of selected lights, and simultaneously therewith to suppress side lobes, to thereby obtain excellent filter characteristics.
It is possible to realize an operation at a single wave by using the aforementioned multi-wavelength light source. However, in the multi-wavelength light source of the present invention, since the single mode oscillation is suppressed by utilizing the frequency shifts, the operation at a single wave becomes theoretically unstable. In order to obtain oscillation at a single wave, it is desirable that the frequency shifts are conducted within the resonator. For example, the frequency shifts can be cancelled by using an acousto-optic tunable filter of two-staged constitution. However, when the frequency shifts are cancelled, it is certainly possible to obtain the stable oscillation in a single mode, but it becomes rather difficult or impossible in turn to obtain an operation at plural wavelengths.
Thus, as a constitution for realizing a single-wavelength operation and a multi-wavelength operation by electrically switching them, the aforementioned acousto-optic tunable filter of the multi-wavelength light source includes a plurality of acousto-optic tunable filter elements connected to each other in multi-stages, and is capable of selectively driving the acousto-optic tunable filter elements, so as to switch between the multi-wavelength oscillation state in which the frequency shifts of selected lights in the driven acousto-optic tunable filter elements become a value other than zero and the single-wavelength oscillation state in which the frequency shifts of selected lights in the driven acousto-optic tunable filter elements become zero. By using such an acousto-optic tunable filter, it becomes possible to realize a light source capable of electrically switching the multi-wavelength oscillating operation and the single-wavelength oscillating operation.
Further objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments when read in conjunction with the accompanying drawings.